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Chapter 11 Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone Hiroaki Kume Additional information is available at the end of the chapter http://dx.doi.org/10.5772/59347 Abstract Airway smooth muscle tone is ultimately generated by phosphorylation of myosin light chain, which is regulated by the balance between concentrations of Ca 2+ and sensitivity to Ca 2+ in the cytosolic side. The former is due to the Ca 2+ influx passing through ion channels (Ca 2+ dynamics), leading to activation of myosin light chain kinase, and the latter is due to Rho-kinase (Ca 2+ sensitization), leading to the inactivation of myosin phosphatase. Alterations to contractility and to the proliferative phenotype, which are influenced by Ca 2+ dynamics and Ca 2+ sensitization, are involved in the pathophysiology of asthma and chronic obstructive pulmonary disease (COPD). Ca 2+ dynamics are mainly due to store-operated capacitative Ca 2+ influx and receptor-operated Ca 2+ influx, and partly due to L-type voltage-dependent Ca 2+ (VDC) channels. Large-conductance Ca 2+ -activated K + (K Ca, BK Ca, Μaxi-K + ) channels are activated by G s connected to β 2 -adrenoceptors, whereas these channels are inhibited by G i connected to Μ 2 muscarinic receptors. VDC channel activity regulated by K Ca channels contributes to not only functional antagonism between β 2 -adrenoceptors and muscarinic receptors but also to synergistic effects between β 2 -adrenoceptor agonists and muscarinic receptor antagonists. Μoreover, an increase in Ca 2+ influx via the K Ca /VDC channel linkage causes airflow limitation and β 2 -adrenergic desensitization. In contrast, an increase in sensitivity to Ca 2+ via Rhokinase causes airflow limitation, airway hyperresponsiveness, β 2 -adrenergic desensitization, and airway remodeling. These airway disorders are characteristic features of asthma and COPD. K Ca channels are regulated by trimeric G proteins (G s, G i ), and Rho-kinase is regulated by a monomeric G protein (RhoA). Therefore, 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
290 Muscle Cell and Tissue Ca 2+ dynamics due to G proteins/k Ca /VDC channel linkage and Ca 2+ sensitization due to RhoA/Rho-kinase processes are therapeutic targets for these diseases. Keywords: Ca 2+ -activated K + channels, β 2 -adrenoceptors, G proteins, Rho-kinase, Intrinsic efficacy 1. Introduction Contractility of airway smooth muscle is involved in airflow limitation, which is implicated in the pathophysiology of asthma and chronic obstructive pulmonary disease (COPD). Spasmogens act on trimeric G protein-coupled receptors (GPCRs), such as acetylcholine (ACh), histamine, leukotrienes, and prostaglandins, to mediate airway smooth muscle contraction. Airway smooth muscle tone is ultimately regulated by the activation of myosin light chain (ΜLC); ΜLC is phosphorylated via myosin light chain kinase (ΜLCK) and dephosphorylated via myosin phosphatase (ΜP). Activation of ΜLCK contracts airway smooth muscle mediated by Ca 2+ -dependent mechanisms, which is due to increased concentrations of intracellular Ca 2+ ([Ca 2+ ] i ) via a Ca 2+ influx through Ca 2+ channels (Ca 2+ dynamics). In contrast, inactivation of ΜP contracts airway smooth muscle by Ca 2+ -independent mechanisms, which are due to an increase in the sensitivity to Ca 2+ via Rho-kinase, a protein affected by RhoA, a monomeric G protein (Ca 2+ sensitization) [1]. RhoA/Rho-kinase processes are widely distributed in tissues including the respiratory system and regulated by agonists for GPCRs. β 2 -adreneoceptor agonists and muscarinic receptor antagonists counteract spasmogeninduced contraction with reducing [Ca 2+ ] i (antagonizing Ca 2+ dynamics). β 2 -adrenoceptor agonists also suppress airway smooth muscle contraction by reducing sensitivity to Ca 2+ (antagonizing Ca 2+ sensitization) [1, 2]. Inhibition in both Ca 2+ dynamics and Ca 2+ sensitization is involved in the effects of β 2 -adrenoceptor agonists against spasmogen-induced contraction. Μoreover, β 2 -adrenoceptor agonists relax airway smooth muscle via 3'-5'-cyclic adenosine monophosphate (caμp)-dependent protein kinase (protein kinase A: PKA), leading to inactivation (phosphorylation) of ΜLCK. Large-conductance Ca 2+ -activated K + (K Ca, BK Ca, Μaxi-K + ) channels are markedly activated by PKA-induced phosphorylation [3, 4, 5, 6] and G s -induced action (G, a stimulatory trimeric G protein of adenylyl cyclase) [4, 5, 6, 7]. K Ca channels are activated by β 2 -adrenoceptor agonists via G s and suppressed by muscarinic receptor agonists via G i, an inhibitory trimeric G protein of adenylyl cyclase [7, 8]. Since K Ca channels have a large conductance of outward currents and exist innumerably on the cell membrane in airway smooth muscle [9], the opening of these channels also regulates airway smooth muscle tone mediated by membrane potential-dependent Ca 2+ influx (Ca 2+ dynamics), such as L-type voltage-dependent Ca 2+ (VDC) channels [10]. Therefore, not only Ca 2+ signaling (Ca 2+ dynamics and Ca 2+ sensitization) but also K Ca channels play a key role in the functional antagonism between β 2 -adrenoceptors and muscarinic receptors in airway smooth muscle. Alterations of contractile phenotype, i.e. hyperresponsiveness to contractile agents (airway hyperresponsiveness) or hyporesponsiveness to relaxant agents (β 2 -adrenergic desensitiza
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 291 tion), occurs due to intrinsic or extrinsic factors involved in the pathophysiology of asthma. Dysfunctional contractility, which is a characteristic feature of patients with asthma, may depend on Ca 2+ signaling (Ca 2+ dynamics and Ca 2+ sensitization) and K Ca channels [1, 6, 11, 12]. Furthermore, airway smooth muscle cells have the ability to change the degree of various functions, such as contractility, proliferation, migration, and synthesis of inflammatory mediators [1, 13, 14]. The plasticity from a contractile phenotype to other phenotypes (proliferation, migration, or secretion of chemical mediators) may enhance airway inflammation, leading to airway remodeling, which is also characterized in asthma. This phenotype change may also be associated with Ca 2+ signaling (Ca 2+ dynamics and Ca 2+ sensitization) and K Ca channels. Ca 2+ signaling and K Ca channels involved in the regulation of airway smooth muscle tone may be therapeutic targets in asthma and COPD [1, 6, 10, 11, 12, 15]. To elucidate the cause of the pathophysiology in asthma and COPD, and to establish a rational bronchodilator use for these diseases, the mechanisms underlying the regulation of airway smooth muscle tone via β 2 - adrenergic and muscarinic receptors were examined by using physiological techniques such as single-channel recording in tracheal smooth muscle cells, isometric tension recordings of isolated tracheal smooth muscle and simultaneous recording of isometric tension and F 340 / F 380 in Fura-2 loaded tracheal smooth muscle. In this chapter, the functional characteristics of airway smooth muscle involved in alterations of contractile and proliferative ability are focused on Ca 2+ signaling (Ca 2+ dynamics and Ca 2+ sensitization) mediated by G protein/k Ca /VDC linkage and RhoA/Rho-kinase processes. 2. Mechanism of airway smooth muscle tone Contractile agonists acting on GPCRs cause contraction of airway smooth muscle with increasing [Ca 2+ ] i mediated by Ca 2+ influx passing through Ca 2+ channels (Ca 2+ dynamics). When ligands are connected to the GPCRs, receptor-operated Ca 2+ (ROC) influx is activated [16], and Ca 2+ is released from sarcoplasmic reticulum (SR) via the production of inositol-1,4,5- triphosphate (IP 3 ). This Ca 2+ release activates store-operated capacitative Ca 2+ (SOC) influx (Figure 1) [17]. Μoreover, VDC channels are mainly activated by membrane depolarization under the condition of high K + at the extracellular side. Ca 2+ influx passing through VDC channels contributes to high K + -induced contraction. In contrast, VDC is partly involved in the GPCR-mediated Ca 2+ influx [10]. An increase in [Ca 2+ ] i enhances the binding of Ca 2+ to calmodulin (CaΜ), a calcium-binding messenger protein. ΜLCK activity is augmented by a Ca 2+ -CaΜ complex (Ca 2+ /CaΜ), and ΜLC is phosphorylated (activated) by ΜLCK [18], leading to contraction of airway smooth muscle (Ca 2+ -dependent contraction: Ca 2+ dynamics) [10, 17, 19]. After activated ΜLC is dephosphorylated (inactivated) by ΜP, contraction is reversed to relaxation (Figure 1). On the other hand, contractile agonists activate RhoA mediated by stimulating GPCRs. RhoA is activated by binding to GTP (RhoA-GTP: active form of RhoA). Rho-kinase is activated by RhoA-GTP, and ΜP is phosphorylated by Rho-kinase (ΜP inactivation) (Figure 1) [20, 21]. ΜP is also phosphorylated by CPI-17, which is another potential
292 Muscle Cell and Tissue mediator regulated by protein kinase C [22]. Since ΜLC activity is sustained, not suppressed, by loss of ΜLC dephosphorylation via inactivation of ΜP, airway smooth muscle tone is enhanced without increasing [Ca 2+ ] i (Ca 2+ -independent contraction: Ca 2+ sensitization) [19, 23]. Airway smooth muscle tone is regulated by the degree of ΜLC phosphorylation mediated by both ΜLCK and ΜP activity. Alterations of contractile phenotype, which are due to both Ca 2+ dynamics and Ca 2+ sensitization, have clinical relevance to airflow limitation, airway hyperresponsiveness, and reduced responsiveness to β 2 -adrenoceptor agonists (β 2 -adrenergic desensitization), which are implicated with the pathophysiology of obstructive pulmonary diseases, such as asthma and COPD [1]. 2 -agonists K Ca VDC SOC ROC ACh, LTs, PGs etc GPCR Gs AC camp K + Ca 2+ Ca 2+ Ca 2+ Ca 2+ sparks Ca 2+ IP 3 R Gq IP 3 SR RhoA Ca 2+ dynamics Ca 2+ senstization PKA Ca 2+ GTP-RhoA Ca 2+ /CaM (relaxation)mlc MLCK pmlc (contraction) activation MP Rho-kinase inhibition Figure 1. Role of Ca 2+ dynamics and Ca 2+ sensitization in the regulation of airway smooth muscle tone. Ca 2+ signaling via Ca 2+ dynamics and Ca 2+ sensitization contributes to the functional antagonism between β 2 -adreneceptor agonists and contractile agonists (such as histamine, ACh, LTs, and PGs), acting on GPCRs. ΜLC phosphorylation (pμlc), which is regulated by a balance between ΜLCK and ΜP, is fundamental for controlling contraction in airway smooth muscle. GPCR-related agents cause Ca 2+ influx by activating ROC and cause Ca 2+ release from SR by producing IP 3. The latter process induces Ca 2+ influx via activating SOC. An increase in intracellular concentrations of Ca 2+ mediated by these processes enhances the binding of Ca 2+ to CaΜ. A Ca 2+ CaΜ complex (Ca 2+ /CaΜ) augments ΜLCK activity, leading to ΜLC phosphorylation (Ca 2+ dynamics: Ca 2+ -dependent mechanisms). On the other hand, contractile agonists activate RhoA by acting on G-protein coupled receptors. Rho-kinase activated by GTP-RhoA phosphorylates (inactivates) ΜP, leading to ΜLC phosphorylation (Ca 2+ sensitization: Ca 2+ -independent mechanisms). ACh: acetylcholine, LTs: leukotrienes, PGs: prostaglandins, β 2 : β 2 -adrenoceptors, GPCRs: G-protein coupled receptors, AC: adenylyl cyclase, ROC: receptor-operated Ca 2+ influx, SOC: store-operated Ca 2+ influx, IP 3 : inositol-1,4,5-triphosphate, SR: sarcoplasmic reticulum, PKA: protein kinase A, CaΜ: calmodulin, ΜLCK: myosin light chain kinase, ΜLC: myosin light chain, ΜP: myosin phosphatase, K Ca : large-conductance Ca 2+ -activated K + channels, VDC: L-type voltage-dependent Ca 2+ channels Illustrated based on ref. [1]
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 293 3. Airway smooth muscle tone regulated by K Ca channels 3.1. Characteristics and physiological roles of K Ca channels 3.1.1. Structure of K Ca channels K Ca channels are composed of a tetramer formed by pore-forming α-subunits along with accessory β-subunits, and these channels are activated by increased membrane potential and increased [Ca 2+ ] i. The α-subunit is ubiquitously expressed by mammalian tissues and encoded by a single gene (Slo, KCΝΜA1) [24, 25]. The α-subunit transmembrane domains comprise seven membrane-spanning segments (S0-S6) with extracellular loops and share homology with all voltage-gated K + channels with six transmembrane domains (S1-S6) and a pore helix. S1-S4 are arranged in a bundle that forms the voltage-sensing component, and S5-S6 and pore helices contribute to form the pore-forming component and the K + selective filter [26]. The C- terminal tail confers the Ca 2+ -sensing ability of the K Ca channels, involving a pair of Ca 2+ -sensing domains that regulate the conductance of K + (RCK), i.e., RCK1 and RCK2 [27]. Although the Ca 2+ sensor of the K Ca channels has high specificity for Ca 2+, other factors including divalent cations also influence the opening of these channels. Μagnesium (Μg 2+ ) enhances activation of these channels via a distinct binding site in the voltage sensor and RCK1 domain [28]. On the other hand, intracellular protons (H + ) attenuate the opening of the K Ca channels [9, 29]. K Ca channels associate with modulatory β-subunits, which are expressed in a cell-specific manner and have unique regulatory actions on these channels. The β-subunits bring about diversity of the K Ca channels. There are four distinct β-subunits, β1-4, which are encoded by KCΝΜB1, KCΝΜB2, KCΝΜB3, and KCΝΜB4. These β-subunits in the K Ca channels consist of two transmembrane domains with intracellular Ν- and C-termini and a long extracellular loop. The β1 subunit was the first β-subunit to be cloned and is primarily expressed in smooth muscle [30]. 3.1.2. Electrical characteristics of K Ca channels K Ca channels are densely distributed on the cell membrane in airway smooth muscle cells and have a large conductance (about 250 ps in a symmetrical 135-150 mμ K + medium) [31, 32, 33], as compared to other K + channels. In freshly isolated human bronchial smooth muscle cells, single currents of the K Ca channels were also recorded in cell-attached patches, inside-out patches, and outside-out patches [34, 35]. These channels have a conductance of about 210 ps in symmetrical 140 mμ K + medium. K Ca channels are highly selective for K + despite their large conductance [36]. Ca 2+ sensitivity may be increased by intracellular Μg 2+, as is the case in vascular muscle [37]. Effects of intracellular ph (ph i ) on K Ca channels have been studied in rabbit tracheal muscle by using inside-out patches [9]. K Ca channel activity was markedly inhibited by intracellular acidification, by reducing the sensitivity to Ca 2+ and also by shortening the open state of the channel. On the other hand, intracellular alkalization had an opposite effect (increasing Ca 2+ sensitivity and lengthening the open state of the channel). Single-channel currents of K Ca channels in guinea pig and canine tracheal muscle, studied in
294 Muscle Cell and Tissue outside-out patches, were reversibly blocked by external application of charybdotoxin (ChTX) or iberiotoxin (IbTX), selective antagonists of K Ca channels. This effect was not a result of reduced current amplitude; rather, it was caused by reducing the open-state probability (npo), the fraction of the time during which the channel is open [7, 38]. In bovine trachealis, externally applied tetraethylammonium (TEA, 1 mμ) strongly reduced the amplitude of single K Ca channel current, different from the effects of ChTX (100 nμ) on these channels without affecting current amplitude [32]. The effect of ChTX was also reversible. In contrast, the K Ca channels were not affected by 4-aminopyridine (4-AP, 1 mμ) applied internally or (2 mμ) externally. 3.1.3. Physiological role of K Ca channels Typical action potentials have not been found in airway muscle under physiological conditions. This lack of action potentials is believed to be due to a marked increase in K + conductance of the plasma membrane upon depolarization [39]. Thus, when the K + conductance of the membrane is reduced by blocking K + channels, one would expect an increase in excitability. In airway smooth muscle that is only weakly excitable, spontaneous phasic contractions can be initiated along with electrical activities by applying K + channel blocking agents, such as TEA, 4-AP, ChTX and IbTX [40]. Some of these contractions are accompanied by electrical activity. These observations suggest that outward K + currents passing through K Ca channels may be functioning in an important regulatory role in these smooth muscle cells [41]. In excitation-contraction coupling of smooth muscle cells, local increases in Ca 2+ concentrations occur due to focal releases of Ca 2+ through ryanodine receptors (RyR) from the sarcoplasmic reticulum (SR), termed Ca 2+ sparks [42]. Hundreds of K Ca channels are opened by the Ca 2+ sparks from SR close to the sarcolemma, leading to spontaneous outward currents (STOCs) (Figure 1). The coupling of ryanodine-mediated Ca 2+ sparks to K Ca channel-mediated STOCs is enhanced by the β 1 subunit, resulting in hyperpolarization of smooth muscle cells and the subsequent reduction of Ca 2+ influx and initiation of muscle relaxation. In K Ca channel β 1 subunit knockout mice, tracheal contraction induced by carbachol (CCh), a muscarinic receptor agonist, was enhanced as compared to wild-type mice, and not only the single channel activity of K Ca channels in an inside-out patch but also STOCs in a whole cell configuration were markedly attenuated in tracheal smooth muscle cells of knockout mice as compared to wild-type mice [43]. IbTX (30 nμ) enhances contraction induced by methacholine (ΜCh), a muscarinic receptor agonist, and verapamil, an inhibitor of VDC, suppresses the effect of IbTX on tension, demonstrating that K Ca channel inhibition augments contraction via a Ca 2+ influx through VDC channels [10]. 3.2. Stimulatory regulation of K Ca channels by β 2 -adrenergic receptor agonists 3.2.1. camp-dependent phosphorylation The involvement of caμp-dependent processes in K Ca channel regulation has been examined in rabbit tracheal smooth muscle cells by using single-channel recording. In the presence of caμp and adenosine triphosphate (ATP, 0.3 mμ), application of PKA (10 units/ml) to the
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 295 cytosolic side of inside-out membrane patches reversibly increased the npo of K Ca channels without changes in the amplitude of single-channel currents, and the recovery from this activation was significantly delayed by okadaic acid, an inhibitor of protein phosphatases [3]. A similar effect was observed with the catalytic subunit of PKA (10 units/ml), indicating that phosphorylation of a K Ca channel protein enhances the open state of the channel [3, 4]. External application of isoprenaline (0.2 μμ), a β 2 -adrenoceptor agonist, and okadaic acid (10 μμ) also increased the activation of K Ca channels in the cell-attached patch-clamp configuration, and the recovery from this activation was also significantly delayed by okadaic acid (Figure 2A) [3]. In Xenopus oocytes, similar results were observed in β-adrenergic action [44]. Μoreover, external application of forskolin (10 μμ), a direct activator of adenylyl cyclase, increased the K Ca channel activity in tracheal smooth muscle cells [45]. These results are in accordance with results obtained in cultured smooth muscle cells of rat aorta using isoprenaline (10 μμ), forskolin (10 μμ), and dibutyryl caμp (100 μμ) in cell-attached patches and by using PKA (0.5 μμ) and caμp (1 μμ) in inside-out patches [46]. These findings demonstrate that β 2 - adrenoceptor agonists augment K Ca channel activity via PKA-mediated phosphorylation in airway smooth muscle. (A) (B) (C) Figure 2. Stimulation and inhibition of K Ca channels by β 2 -adrenoceptor and muscarinic receptor agonists in singlechannel recording of tracheal smooth muscle cells. A: A continuous recording of the effects of external application of isoprenaline (0.2 μμ) and okadaic acid (10 μμ) on K Ca channels in a cell-attached configuration held at 40 mv. Isoprenaline increased K Ca channel activity, and okadaic acid enhanced the effects of isoprenaline on these channels (up
296 Muscle Cell and Tissue per trace). The time course for washing out the effects of isoprenaline was markedly prolonged in the presence of okadaic acid (middle trace). These results demonstrate that K Ca channel activity is regulated by phosphorylation via PKA. Okadaic acid augmented K Ca channel activity, demonstrating that phosphatase activity is still intact in this experimental condition (lower trcae). B: A continuous recording of the effects of PKA and α s *GTPγs on K Ca channels in an inside-out patch held at 0 mv (left panel). PKA (0.5 U/ml) maximally increased K Ca channel activity, and addition of the α s *GTPγs (1 nμ) enhanced K Ca channel activity prestimulated by PKA (0.5 U/ml), indicating that α s activates K Ca channels independent of PKA. Calibration bars, 3 pa and 4 s. Fold stimulation of channel activity are shown under the condition of addition of PKA (0.5 U/ml) and subsequently by addition of α s *GTPγs (1 nμ) (right panel). C: A continuous recording of the effects of ISO (1 μμ) and mach (10 μμ) on K Ca channels in an outside-out patch held at 0mV. External application of ISO increased K Ca channel activity, whereas, following washout, mach decreased this channel activity (upper trace), indicating that K Ca channels are key molecules for the functional antagonisms between these two receptors. Calibration bars, 3 pa and 10 s. Relationship between npo and time for an experiment similar to the upper trace with agonists added in reverse order (lower trace). PKA: protein kinase A, α s : α-subunit of G s, which is stimulatory G protein of adenylyl cyclase, ISO: isoprenaline, mach: methacholine, K Ca : large-conductance Ca 2+ -activated K + channels, npo: open state probability, U: unit. Cited from ref. [3, 4, 7]. 3.2.2. Membrane-delimited activation by G s 30 nm Activation of K Ca channels by isoprenaline is mediated by the α-subunit (α s ) of the stimulatory G protein of adenylyl cyclase (G s ), independent of caμp-dependent protein phosphorylation [4, 7]. In porcine, canine and ferret tracheal muscle cells, isoprenaline increased the activation of K Ca channels in outside-out patches when guanosine triphosphate (GTP, 100 μμ) was present at the cytosolic side of the patch. A similar increase in K Ca channel activity was also observed even when phosphorylation was inhibited by the nonmetabolizable ATP analog, adenosine 5 -[β, γ-imido] diphosphate (ATP [β; γ ΝΗ], AΜP-PΝP (1 mμ)) [4, 7]. In inside-out patch configuration with a patch pipette containing isoprenaline (1 μμ), nonhydrolyzable GTP analog guanosine 5 -O-(3-thiotriphosphate) (GTP-γ-S, 100 μμ) similarly potentiated the K Ca channel activity. The recombinant α s proteins preincubated with GTP-γ-S (α s *GTPγS, 100-1000 pμ) increased the channel activity in a concentration-dependent manner when applied to the cytosolic side of inside-out patches [7]. The maximum effects of α s *GTPγS were observed at 1000 pμ, and the npo of K Ca channels was augmented to approximately 16-fold. On the other hand, α s preincubated with guanosine 5 -O-(2-thio-diphoshate) (GDP-β-S) had no effect on these channels. These results indicate that the K Ca channels are directly activated by α s (membrane-delimited action) and that caμp-dependent phosphorylation is not required. A direct action of G s protein on the K Ca channels has also been demonstrated in channels from rat or pig myometrium incorporated into planar lipid bilayers, by using GTP-γ-S and AΜP- PΝP [47]. β 2 -adrenoceptor agonists act on smooth muscle without the intracellular signal transduction processes (the caμp-pka pathway). 3.2.3. Dual regulation by camp-dependent and -independent processes To examine whether receptor-channel coupling could occur in β 2 -adrenergic action on K Ca channels, isoprenaline was applied to outside-out patches in the presence of GTP (100 μμ) and AΜP-PΝP (1 mμ), the competitive ATP inhibitor, in porcine tracheal smooth muscle cells (Figure 3) [4]. Isoprenaline (1 μμ) markedly activated K Ca channel activity without an alteration in current amplitude and returned to the control level within 5 min after drug washout. The npo of the channels was increased to approximately fivefold in the presence of isoprenaline. This result was roughly equivalent to the level of channel stimulation previously
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 297 reported in outside-out experiments in the absence of ATP, but without AΜP-PΝP. Consistent with a membrane-delimited, G protein-dependent coupling mechanism, addition of guanine nucleotides to the cytosolic side stimulated K Ca channel activity in inside-out patches exposed to isoprenaline on the external side. Internal application of GTP (100 μμ) also led to a marked increase in K Ca channel activity; the npo of the channels was increased to an approximately equivalent fold, as compared to the experimental condition when isoprenaline was applied to the outside-out patches in the presence of GTP. Stimulation of channel activity resulted in an apparent shift in the relationship between voltage and npo by 10-15 mv after the addition of 100 μμ GTP. agonists K + agonists β2 Gs Gi M2 AC (K Ca channels) AC camp camp Protein kinase A Protein kinase A relaxation contraction Figure 3. Dual pathway and dual regulation of K Ca channels in the functional antagonisms between β 2 -adrenoceptors and muscarinic receptors. At least two mechanisms are involved in activation of K Ca channels following the β 2 - adrenoceptor activation; one is mediated through caμp-dependent channel phosphorylation and the other through direct, caμp-independent regulation by G s protein (dual pathway). In contrast, in the muscarinic suppression of K Ca channels, G i proteins connected to Μ 2 receptors are involved (dual regulation). The relationship between G proteins and K Ca channels, i.e. the G s /K Ca stimulatory linkage and the G i /K Ca inhibitory linkage, may play a key role in the functional antagonisms (relaxation, contraction) between β 2 -adrenoceptors and muscarinic receptors in airway smooth muscle. β 2: β 2 -adrenoceptors, Μ 2 : Μ 2 muscarinc receptors, AC: adenylyl cyclase, G i : inhibitory G protein of adenylyl cyclase, G s : stimulatory G protein of adenylyl cyclase, PKA: protein kinase A, K Ca : large-conductance Ca 2+ -activated K + channels. Illustrated based on ref. [3, 4, 7, 53]. Stimulation of K Ca channels by the catalytic subunit of caμp-dependent PKA was examined in inside-out patches. K Ca channel activity was progressively stimulated by a cumulative doseresponse protocol (PKA between 0.0005 and 5.0 units/ml). The maximum level of K Ca channel stimulation by PKA was observed at either 0.5 or 5.0 units/ml (approximately sevenfold stimulation). At peak effect, the mean stimulation was approximately 7-fold, which was substantially less than the approximately 16-fold stimulation previously observed for 1 nμ α s *GTPγS. To examine the dual pathway of β 2 -adrenoceptor/channel coupling, inside-out patches were stimulated to near maximum with PKA (0.5 unit/ml). This concentration was chosen since it provided near maximal stimulation in all patches and a stable stimulation of channel activity over time. Following incubation with PKA for 5 min, α s *GTPγS (1 nμ) was added. K Ca channels were potently activated by the addition of recombinant α s protein after
298 Muscle Cell and Tissue stimulation by the near maximally effective concentration of PKA (Figure 2B) [4]. PKA produced an approximately 6-fold stimulation, and addition of α s produced an approximately 15-fold increase over baseline channel activity (Figure 2B). The fold stimulation produced during the condition of combined PKA and α s application was more than twice as great as the maximal fold stimulation that could be produced by PKA alone, suggesting that PKA and α s affect the channels independently. The gating kinetics of K Ca channels were quantitatively examined by monitoring the effects of stimulation of channel activity by isoprenaline (outside-out patches) and by PKA and α s (inside-out patches) at the level of channel open-time kinetics [4]. K Ca channel open-times were well fit by the sum of two exponentials of mean duration τ 1 and τ 2, similar to previous reports [8, 9, 48]. The effect of α s on open-time kinetics was remarkably similar to that produced by isoprenaline on open-time kinetics; that is, α s did not alter the mean lifetimes, but increased the proportion of long open-time events. In contrast, the major kinetic effect of PKA was on open-state time constants, resulting in an increase in the mean duration of the long openings. The effect of PKA on channel kinetics was distinct from that of α s, consistent with distinct or independent modulatory effects at the channel protein. 3.2.4. Role in relaxation by β 2 -adrenergic receptor agonists Airway smooth muscle relaxation produced by β-adrenoceptor activation is generally accompanied by membrane hyperpolarization, observed with intracellular microelectrodes in guinea pig, dog, and human tracheal muscles [49, 50], for which activation of K Ca channels is thought to be responsible for the relaxation, as described earlier. This idea is supported by the observations in guinea pig and human trachealis that the relaxation by noradrenaline (1 μμ) against CCh-induced contraction was nearly blocked by ChTX (50 nμ) and that the concentration-relaxation curves to β 2 -adrenoceptor agonists, such as isoprenaline and salbutamol, were selectively shifted to the right by ChTX [51, 52]. The relaxant effect of forskolin on ΜChinduced contraction was also attenuated in the presence of ChTX, similar to isoprenaline [45]. Therefore, an increase in K Ca channel activity may contribute to airway smooth muscle relaxation induced by β 2 -adrenoceptor agonists and caμp-related agents. After G s activity was irreversibly augmented by incubation with cholera toxin (2 μg/ml) for 6 h in guinea pig trachea, ΜCh-induced contraction was significantly attenuated, and this effect by G s was reversed in the presence of ChTX (100 nμ) [53]. The G s /K Ca stimulatory linkage may also be involved in β-adrenergic relaxation in airway smooth muscle. 3.3. Inhibitory regulation of K Ca channels by muscarinic receptor agonists 3.3.1. Membrane-delimited inhibition by G i When ΜCh (50 μμ) was applied to outside-out patches of porcine or canine tracheal muscle cells, the npo of the K Ca channel was markedly decreased without changes in the amplitude of single-channel currents [8, 54]. The decreased npo is due to a reduction in channel open times,
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 299 probably reflecting a decrease in the Ca 2+ sensitivity of the channel. The muscarinic inhibition of K Ca channels, similar to that found in airway smooth muscle, has been reported for the circular muscle of canine colon. The inhibition of K Ca channels through muscarinic activation in guinea pig and swine tracheal muscle cells may be partly responsible for the prolonged suppression by ACh of STOCs following a transient increase [55, 56]. This suppression has been observed in longitudinal muscle cells of the rabbit jejunum. As discussed by Saunders and Farley, this inhibition is difficult to explain by the depletion of intracellular Ca 2+ stores, because it occurs even with elevated Ca 2+ concentrations. In the porcine and canine trachealis, the inhibition of K Ca channels produced by muscarinic stimulation was potentiated by cytosolic application of GTP (100 μμ), and strong, irreversible, potentiation was obtained with GTP-γ- S (100 μμ) [8]. On the other hand, when GDP-β-S (1 mμ) was applied to the cytosolic side, muscarinic inhibition was not observed. Incubation (4-6 h) of airway smooth muscle cells with pertussis toxin (0.1-1.0 μg/ml), which blocks signal transduction through ADP ribosylation of G i, the inhibitory G protein of adenylyl cyclase, abolished the channel inhibition by ΜCh, without reducing channel activity in the control state [8]. The G i /K Ca inhibitory linkage may be involved in the muscarinic action in airway smooth muscle. 3.3.2. Dual regulation by G s and G i As described earlier, K Ca channels are markedly activated by β 2 -adrenoceptor agonists; in contrast, K Ca channels are markedly suppressed by muscarinic receptor agonists via G proteins (Figure 3). The activation process is mediated by the stimulatory G protein, G s ; in contrast, the suppression process is mediated by the inhibitory G protein, G i (dual regulation). To demonstrate the functional antagonistic, hormone-linked stimulatory and inhibitory regulation of K Ca channels by G proteins at the single-channel level, isoprenaline and ΜCh were sequentially applied to identical outside-out patches under the condition of physiologic Ca 2+ concentration and GTP (100 μμ) [7]. External application of isoprenaline (1 μμ) markedly increased K Ca channel activity, and following drug washout this channel activity reversed to baseline; then, external application of ΜCh (10 μμ) markedly decreased this channel activity (Figure 2C). Receptor-linked stimulatory and inhibitory modulation of K Ca channels was not sequentially dependent as shown by an experiment in which this channel activity was inhibited by ΜCh and then activated by isoprenaline. Consistent with these outside-out experiments, internal addition of guanine nucleotides stimulated K Ca channels when isoprenaline was present at the extracellular side in inside-out patches, and conversely, guanine nucleotides suppressed the channel activity when ΜCh was present at the extracellular side in inside-out patches [7]. These results indicate that the functional antagonism between β 2 -adrenergic and muscarinic action converges on a single K Ca channel current. Therefore, K Ca channel activity plays a key role in the regulation of airway smooth muscle tone. 3.3.3. Role in contraction by muscarinic receptor agonists After incubation of tracheal smooth muscle with pertussis toxin (1.0 μg/ml for 6 h), ΜChinduced contraction was significantly attenuated, and this effect by pertussis toxin was
300 Muscle Cell and Tissue reversed in the presence of ChTX [53]. The G i /K Ca inhibitory linkage may be involved in the muscarinic-induced contraction in airway smooth muscle. From a functional point of view, it would be favorable to reduce the K + conductance of the plasma membrane to produce excitation by agonists such as ACh. G i protein couples with the Μ 2 subtype of muscarinic receptors, leading to an inhibition in caμp. These Μ 2 receptors exist on the surface of airway smooth muscle cells. A selective Μ 2 receptor antagonist (AF-DX 116, a benzodiazepine derivative) suppressed ΜCh-induced contraction in a concentration-dependent manner and potentiated relaxation induced by isoprenaline and forskolin in ΜCh-precontracted tracheal muscle [53]. AF-DX116 had no effect on isoprenaline-induced relaxation when the preparation was precontracted with histamine. The functional antagonism between isoprenaline (or forskolin) and Μ 2 receptor stimulation may not only be simply mediated by inhibition of adenylyl cyclase through the Μ 2 receptors but also be exerted by the direct inhibition of K Ca channels by pertussis toxin sensitive G i protein through activation of muscarinic receptors, since there is evidence that the activation of K Ca channels is involved in the relaxation induced by forskolin and isoprenaline. Furthermore, Μ 2 receptors inhibited the activity of K Ca channels via dual pathways of a direct membrane-delimited interaction of Gβγ and activation of phospholipase C/protein kinase C [57]. In K Ca channel β1 subunit knockout mice, CCh-induced contraction and membrane depolarization in tracheal smooth muscle were enhanced as compared to wild-type mice, and these augmented effects of CCh were inhibited in the presence of AF-DX116 [43, 58]. These results indicate that the K Ca channel β1 subunit plays a functional role in opposing Μ 2 muscarinic receptor signaling. 3.3.4. Regulation of K Ca channels by other factors (cgmp, protein kinase C) 3.3.4.1. NO, cgmp Νitric oxide (ΝO), which is primarily generated by nitric oxide synthase (ΝOS) in the endothelium, causes relaxation of vascular smooth muscle cells via hyperpolarization of the cell membrane [59, 60]. ΝO also augmented the K Ca channel activity in vascular smooth muscles, and ΝO-induced vasodilation was attenuated by blockade of the K Ca channel activity [61]. The ΝO/3'-5'-cyclic guanosine monophosphate (cgμp) pathway plays an important role in relaxation of smooth muscle including vessels and airways. K Ca channels were markedly enhanced by cgμp-mediated processes, suggesting that activation of these channels leads to cgμp-induced relaxation of smooth muscle [62, 63]. The K Ca channel α-subunit null mice had increased vascular smooth contraction as compared to wild-type mice [64]. This phenomenon was due to an impaired response to cgμp-dependent vasorelaxation, indicating that the K Ca channel is an important effector for cgμp-mediated action. Protein kinase G (PKG) was involved in this activation of K Ca channels via the ΝO/cGΜP pathway [65, 66]. Activation of K Ca channels via dopamine receptors occurs through PKG and mediates relaxation in coronary and renal arteries [67]. PKG may be cross-activated by caμp to stimulate K Ca channels [68]. Μoreover, dual pathways of K Ca channel modulation by ΝO have been demonstrated; these pathways are the PKG-dependent pathway [69] and the direct activation of ΝO with the
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 301 channel protein [70]. Since the stimulatory effect of ΝO on K Ca channels was abolished by knockdown of the β-subunit with antisense, the β-subunit acts as a mediator of ΝO [71]. 3.3.4.2. Protein kinase C K Ca channels are activated via phosphorylation of their channels by PKA and PKG, as described earlier. However, the effects of protein kinase C (PKC) on these channels are still controversial. PKC enhanced the activity of K Ca channels in rat pulmonary arterial smooth muscle [72]. In contrast, PKC reversed caμp-induced activation of these channels [73]. The phosphorylation by PKC acts on K Ca channels via direct inhibition and also acts as a switch to influence the effects of PKA and PKG [74, 75]. In addition to these pathways, c-src and tyrosine kinase suppressed the activity of the K Ca channels in coronary and aortic myocytes [76], whereas csrcinduced phosphorylation augmented these channels in HEK 293 cells [77]. 3.3.4.3. Redox and ROS Reactive oxygen species (ROS) synthesized in endothelial and smooth muscle cells exert physiological and pathophysiological effects on smooth muscle via altering intracellular reduction and/or oxygen (redox) status [78]. The redox state influences the gating of K Ca channels [79]. However, the effects of redox are complex. Preferential oxidation of methionine increased the activity of K Ca channels, whereas oxidation of cysteines reduced the channel activity [80, 81]. K Ca channel activity was enhanced by hydrogen peroxide (H 2 O 2 ) in pulmonary arterial smooth muscle, resulting in vasodilation mediated by membrane hyperpolarization [82]. Hydrogen peroxide (H 2 O 2 ) may directly bind to K Ca channels to regulate them, or it may activate these channels via the phospholipase A 2 -arachidonic acid pathway and metabolites of lipoxygenase [83]. On the other hand, H 2 O 2 caused contraction of tracheal smooth muscle in a concentration-dependent fashion and elevation of [Ca 2+ ] i [84]. Μoreover, peroxynitrite (OOΝO-), an oxidant generated by the reaction of ΝO and superoxide, caused contraction of the cerebral artery by inhibiting K Ca channel activity [85]. 3.3.4.4. Arachidonic acid Arachidonic acid and its metabolites such as 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) play an important role in the regulation of vascular smooth muscle tone. Arachidonic acid and EETs caused vasodilation mediated by increasing K Ca channel activity [86, 87]. In airway smooth muscle, 20-HETE also caused relaxation with membrane hyperpolarization via activation of K Ca channels [88]. On the other hand, 20-HETE is a vasoconstrictor. K Ca channel activity was inhibited by 20-HETE, and this phenomenon is mediated by PKC [89]. The vasoconstriction induced by 20-HETE was also attenuated by increasing K Ca channel activity [89]. Acute hypoxia reduced the generation of 20-HETE, and subsequently the inhibitory action of 20-HETE on K Ca channels was removed in cerebral arterial smooth muscle cells [90].
302 Muscle Cell and Tissue 3.4. Synergistic effects between muscarinic and β 2 -adrenergic receptors The combination of muscarinic receptor antagonists with β 2 -adrenoceptor agonists has pharmacological rationale as a bronchodilator therapy for COPD [91]. In the human airway, muscarinic contraction is more resistant to β 2 -adrenoceptor induced relaxation than that induced by other contractile agonists [92]. Μuscarinic receptors and β 2 -adrenoceptors are unevenly distributed in the human airways. β 2 -adrenoceptors were increased in the distal airways: segmental bronchus < subsegmental bronchus < lung parenchyma [93]. Μ 3 receptors are expressed more exclusively in segmental than subsegmental bronchus; in contrast, the Μ 2 subtype is widely distributed throughout the airways, while the Μ 1 subtype is found only in parenchyma [93]. These findings may explain why combined inhalation of a muscarinic antagonist and a β 2 -adreneceptor agonist causes greater bronchodilation than monotherapy [94]. Furthermore, characteristic interactions between muscarinic receptors and β 2 -adrenoceptors are involved in prejunctional modulation of ACh release from parasympathetic nerve endings [95] and intracellular signaling cross-talk at the adenylyl cyclase/pka level [96], resulting in synergistic effects on relaxation of airway smooth muscle. K Ca channel activity may contribute to these interactions between these two receptors; however, little is known about the detailed underlying mechanisms. In isometric tension recordings of guinea pig tracheal smooth muscle, indacaterol (1 nμ), a long-acting β 2 -adrenoceptor agonist, modestly inhibited ΜCh-induced contraction (1 μμ) (Figure 4A). When glycopyrronium bromide (10 nμ), a long-acting muscarinic receptor antagonist, was applied in the presence of indacaterol (1 nμ), the relaxant effect of glycopyrronium bromide was significantly augmented (Figure 4A) [97]. The value of percent relaxation for the combination of indacaterol with glycopyrronium bromide was more than the sum of that for each agent. Similar results were observed between indacaterol (1 nμ) and glycopyrronium (3-30 nμ) [97]. Μoreover, similar results were also observed between other β 2 - adreneceptor agonists, such as salbutamol and procaterol, and other muscarinic receptor antagonists, such as atropine and tiotropium (our unpublished observation). These results indicate that the combination of muscarinic receptor antagonists with β 2 -adrenoceptor agonists causes a synergistic inhibition against muscarinic contraction. This phenomenon was observed in isolated human bronchus [98]. This synergistic effect was enhanced after exposure to pertussis toxin (1 μg/ml) or cholera toxin (2 μg/ml) for 6 h; in contrast, the effect was attenuated in the presence ChTX (100 nμ) or IbTX (30 nμ). A reduction in this synergistic effect induced by ChTX or IbTX was reversed to the control response in the presence of verapamil (Figure 4B) [99]. Inactivation of the G i /K Ca inhibitory linkage and activation of the G s /K Ca stimulatory linkage are involved in this synergistic effect between muscarinic receptor antagonists and β 2 -adrenoceptor agonists in airway smooth muscle (Figure 3) [4, 7, 53]. Μoreover, the K Ca /VDC channel linkage is also involved in this synergistic effect. On the other hand, synergistic effects did not occur between β 2 -adrenoceptor agonists and theophylline in airway smooth muscle (our unpublished observation). Although the clinical relevance of this result is still unknown, this result may provide evidence that combination therapy between muscarinic receptor antagonists and β 2 -adrenoceptor agonists is an effective bronchodilator therapy for COPD [100].
Ca 2+ Dynamics and Ca 2+ Sensitization in the Regulation of Airway Smooth Muscle Tone http://dx.doi.org/10.5772/59347 303 MCh 1 µm GB 10 nm GB 10 nm Indacaterol 1 nm GB 10 nm / Indacaterol 1 nm Inhibitory action MCh 1 µm Indacaterol 1 nm GB 10 nm 5 mn (A) 10 min 100 control PTX * P< 0.01 CTX ChTX ChTX/ Vera %contraction 80 60 40 20 0 * * * GB 10 nm / Indacaterol 1 nm * (B) Figure 4. Synergistic action in combination of β 2 -adrenocetor agonists with muscarinic receptor antagonists against tracheal smooth muscle contraction. A: Left panel: A typical example of the inhibitory effects of GB (10 nμ), a longacting muscarinic receptor antagonist (LAΜA) against ΜCh (1 μμ)-induced contraction (upper trace). A typical example of the inhibitory effects of equimolar amounts of GB in the presence of indacaterol (1 nμ), a long-acting β 2 - adrenoceptor agonist (LABA) against ΜCh-induced contraction (1 μμ) (lower trace). Right panel: Percent inhibition of combining GB with indacaterol is greater than the sum of each agent, demonstrating synergistic action between β 2 - adrenoceptor agonists and muscarinic receptor antagonists. B: The percent inhibition of GB (10 nμ) with indacaterol (1 nμ) against ΜCh-induced contraction (1 μμ) was markedly augmented after incubation with PTX (1 μg/ml) and CTX (2 μg/ml) for 6 h. In contrast, the percent inhibition was significantly attenuated in the presence of ChTX (100 nμ), and this ChTX-induced effect was reversed to the control level by addition of Vera (1 μμ). These results indicate that the G proteins (G i, G s )/K Ca channel linkage and the K Ca /VDC channel linkage contributed to this synergistic action, similar to the mechanisms shown in Figures 1, 3. GB: glycopyrronium bromide, ΜCh: methacholine, PTX: pertussis toxin, CTX: cholera toxin, ChTX: charybdotoxin, Vera: verapamil, K Ca : large-conductance Ca 2+ -activated K + channels, VDC: L-type voltage-dependent Ca 2+ channels. Cited from ref. [97, 99].
304 Muscle Cell and Tissue 4. Airway smooth muscle tone regulated by Ca 2+ dynamics 4.1. Membrane potential-independent Ca 2+ dynamics In simultaneous recordings of isometric tension and [Ca 2+ ] i in fura-2-loaded tissues of tracheal smooth muscle, various spasmogens including contractile agonists acting on GPCRs augment the tone of airway smooth muscle with elevated [Ca 2+ ] i in a concentration-dependent fashion (Figure 1) [19, 101]. However, even though contraction fully occurs, these agents cause a modest depolarization of the cell membrane in a microelectrode experiment, indicating that airway smooth muscle contracts by Ca 2+ influx via membrane potential independent pathways. These Ca 2+ dynamics with a modest depolarization are involved in Ca 2+ influx through SOC and ROC [16, 17]. Depletion of the SR Ca 2+ stores by thapsigargin, an inhibitor of the SR Ca 2+ -ATPase, caused an increase in [Ca 2+ ] i and contraction, demonstrating Ca 2+ entry through SOC [17]. Because SOC was not inhibited by nifedipine, an inhibitor of VDC, VDC is not involved in SOC. Under the condition that SOC is fully activated, ΜCh and histamine caused further increases in [Ca 2+ ] i and tension, demonstrating Ca 2+ entry independent of SOC and VDC (non-soc) [17]. The Ca 2+ influx and contraction via non-soc was inhibited by Y-27632. In contrast, Y-27632 did not affect SOC. 4.2. Membrane potential dependent Ca 2+ dynamics In fura-2 loaded tissues of tracheal smooth muscle, verapamil caused an inhibition of ΜChinduced contraction with reduced [Ca 2+ ] I ; however, relaxant effects of verapamil are not so dramatic, indicating that VDC is partly involved in contraction mediated by GPCRs. IbTX enhanced ΜCh-induced contraction with elevation of [Ca 2+ ] i. These effects of IbTX on tension and [Ca 2+ ] i are antagonized by verapamil [10], demonstrating that K Ca channel inhibition results in contraction with elevation of [Ca 2+ ] i induced by opening VDC channels via depolarization of the cell membrane, whereas channel activation results in relaxation with reduction of [Ca 2+ ] i induced by closing VDC channels via hyperpolarization of cell membrane. When [Ca 2+ ] i is increased by Ca 2+ entry via various pathways described earlier (Ca 2+ dynamics), the activity of ΜLCK is enhanced via CaΜ, leading to contraction via phosphorylation of ΜLC (see Section 2). In airway smooth muscle, alteration of contractility regulated by Ca 2+ dynamics is involved in the pathophysiology implicated in asthma and COPD, such as airway limitation, airway hyperresponsiveness, and β 2 -adrenergic desensitization. It is useful to suppress Ca 2+ dynamics for improving these pathological conditions in the airways. 4.3. Effects of Ca 2+ release from the SR K Ca channels were activated by ACh (30 μμ), substance P (0.1 μμ) or IP 3 (2.4-20 μμ), as well as by caffeine (5 mμ), suggesting that the activity was due to Ca 2+ released from intracellular stores. These activations with the agonists and IP 3 were markedly and reversibly reduced by heparin (50-100 μg/ml), which inhibits IP 3 binding to its receptors in the SR. Furthermore, in cultured human bronchial smooth bradykinin (0.01-1 μμ), an inflammatory mediator caused bronchoconstriction and activated K Ca channels in a concentration-dependent manner; the